Critical behavior of the electrical conductivity near the disorder-induced metal-insulator (MI) transition has been observed in polyaniline (PANI) doped with camphor sulfonic acid (CSA). The temperature dependence of the resistivity (ρ) depends on the degree of disorder present in the PANI-CSA films. In the most metallic samples the resistivity is nearly temperature independent; whereas in the critical region of MI transition, ρ(T) is characterized by a power-law temperature dependence, ρ(T)∝${\mathit{T}}^{\mathrm{-}\mathrm{\beta }}$. Resistivity ratios ρ(1.4 K)/ρ(300 K) as low as 1.6 have been observed in the most metallic samples of PANI-CSA. In the metallic regime, the conductivity is characterized by σ(T)=σ(0)+${\mathit{mT}}^{1/2}$ at low temperatures; the dependence of m(H) on magnetic field provides information on the role of electron-electron interactions in the transport near the MI transition. The ${\mathit{T}}^{\mathrm{-}1}$ dependence found for inelastic-scattering time (${\mathrm{\tau }}_{\mathrm{in}}$) is in agreement with that predicted for metallic systems near the MI transition. For the samples initially in the critical regime, a magnetic field of 8–10 T induces the transition to variable-range hopping. The typical localization length in PANI-CSA just on the insulating side of the MI transition is about 80–130 Å. The magnitude of the positive magnetoresistance increases considerably as the system moves from the metallic to the insulating regime. The magnitude and quasilinear temperature dependence of the thermopower near the critical regime of the MI transition is typical of that expected for a metal.

The electronic structure and the linear-optical properties of the nonlinear-optical crystal β-${\mathrm{BaB}}_2$${\mathrm{O}}_4$ (BBO) have been calculated using a first-principles band-structure method with the local-density approximation. The results are compared with those of another nonlinear-optical crystal ${\mathrm{LiB}}_3$${\mathrm{O}}_5$ (LBO). An indirect band gap of 5.52 eV and a direct gap of 5.61 eV at Γ for BBO are obtained, which are to be compared with the measured optical gap of 6.43 eV. It is shown that the electronic structure of the BBO is dominated by the characteristic ionic (${\mathrm{B}}_3$${\mathrm{O}}_6$${)}^{\mathrm{-}3}$ group in which the two different O sites show markedly different bonding structures. Comparison of charge-density distributions in BBO and LBO shows that the ionic groups are linked together in forming a networklike structure. The Ba atom in the BBO crystal is found to be bonded to the O ions in the adjacent layers in a partly covalent, partly ionic manner due to the presence of large semi-core-like 5p orbitals. The calculated optical properties for both BBO and LBO crystals are in good agreement with the measured vacuum ultraviolet spectrum especially in the region of absorption threshold. The agreements between theory and experiment near the absorption edges are much closer than that reflected by the gap values alone.

The self-consistent generalization of the spline augmented-plane-wave method within a full-potential density-functional framework is presented. The band-structure scheme properly accounts for the k-dependent split-up of the radial wave functions ${\mathit{R}}_{\mathit{l}}$ into ${\mathit{R}}_{\mathit{l}\mathit{m}}$ in the case of an arbitrary potential. As an application we investigate ground-state and related properties of bulk Cu. Full-potential effects turn out to play a minor role. Surprisingly, the absolute value of the equilibrium total energy is found 1.305 Ry below the former result of Moruzzi et al., although the same exchange-correlation potential is employed. The calculated equilibrium values of the lattice constant, bulk modulus, and form factors agree quite satisfactorily with experimental results, while the band energies characteristically deviate from photoemission data. Whenever possible, we compare our results to experimental and previous theoretical data.

The thermally-stimulated-depolarization-current method is used in order to characterize the dielectric relaxation processes in LiF crystals doped with ${\mathrm{Ca}}^{2+}$ impurities. Results on as-grown samples indicate a space charge and/or an interfacial polarization phenomenon. Thermal perturbation is employed to release free impurity-cation vacancy dipoles. The subsequent bands are critically visualized and one of them is attributed to jumps of the bound-cation vacancy via nearest-neighbor paths.

We present a method for calculating the electronic structure of disordered alloys with short-range order (SRO) which guarantees a positive density of states for all values of the SRO parameter. The method is based on the generalized augmented-space theorem which is valid for alloys with SRO. This theorem is applied to alloys with SRO in the tight-binding linear-muffin-tin-orbital (TB-LMTO) framework. This is done by using the augmented-space formulation of Mookerjee and cluster-coherent-potential approximation. As an illustration, the method is applied to a single-band model TB-LMTO Hamiltonian. We find that the SRO can induce substantial changes in the density of states.

The analysis of an earlier paper is extended to calculations of the electronic density of states for potentials with both s and p bound states. An alternative description of the method is given and the formalism developed for a computationally efficient approach. Results are presented showing how the density of states changes as the potential parameters and the structural parameters are varied for a disordered tetrahedrally bonded system. The value of the approach as a basis for extension to calculations of other properties, such as phonons, and to more complex structural systems is discussed.

The theory of electric-dipole two-pulse echo in semiconductors under the conditions of microwave hopping conductivity is developed. It is shown that the echo signal is due to the interaction of an a.c. electric field with pairs of neighboring donors. The characteristic spatial distance between them is much smaller than the average distance. Each such pair consists of one impurity center which is occupied by an electron and another center that is empty. Such a pair (the so-called close pair) is a two-level electron system and different pairs have, in general, different interlevel spacings. The interlevel spacings have random values distributed within a wide range of energies of the order of the impurity bandwidth. The echo pulse form is analyzed as a function of the pumping pulse amplitude for a broad range of amplitudes. The dependence of the echo pulse decay law on the time interval between the pumping pulses is found.

Energy levels of band edges in diluted magnetic semiconductors are calculated in the effective-mass approximation, retaining off-diagonal terms in the exchange interaction and using a plane-wave expansion. This model accounts qualitatively for the observed asymmetry in the splitting of the A exciton in a magnetic field in ${\mathrm{Cd}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$S for which multiple-scattering corrections are expected to be important.

In this work we have developed a kinetics model of energy transfer from the host lattice to the localized core excited states of rare-earth isoelectronic structured traps (REI traps). The presence of low-lying empty core orbitals in rare-earth impurities introduces new excitation and recombination phenomena. To adequately describe the energy transfer to a REI trap, the buildup and decay kinetics of rare-earth luminescence, we consider six separate states of the REI impurity (unoccupied, electron occupied, electron occupied excited, exciton occupied, excited electron occupied, and excited exciton occupied). The energy-transfer processes occur through an Auger mechanism where the recombination energy of the bound electron with a free hole is transferred nonradiatively to the core states, or energy can be transferred from the bound exciton on a REI trap to the core states. If the initial and final states are not resonant (in both mechanisms), the energy mismatch must be accommodated by emission or absorption of phonons. Furthermore we discuss details of several quenching processes, which are incorporated into the kinetics equations. We derive two sets of differential equations for semi-insulating and n-type semiconductors governing the kinetics of rare-earth luminescence. Equations have been solved by a numerical method to derive the time dependence of the rise and decay kinetics as a function of excitation intensity. The numerically simulated luminescence rise and decay times show a good overall quantitative agreement with experimental data obtained for InP:Yb, over a wide range of generation rates.

We present a theoretical study of the effect of partially filled d electrons in transition-metal ions on the ${\mathit{E}}_1$ and ${\mathit{E}}_1$+${\mathrm{\Delta }}_1$ band-gap energies of the diluted magnetic semiconductors. Semiempirical tight-binding calculations were performed to study the effects of interactions arising from the hybridization between the localized d orbitals of a transition-metal impurity and the bulk band states of II-VI semiconductors on the Γ- and L-point band gaps. Our results account for the concentration dependence of the L-point band gaps in the ${\mathrm{Zn}}_{1\mathrm{-}\mathit{x}}$(Mn,Fe,Co${)}_{\mathit{x}}$Se system in the range of small x. The effect of this hybridization strongly depends on the location of the d states relative to the valence- and conduction-band levels, giving a decrease of the L-point band gaps in ${\mathrm{Zn}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$Se, an increase in ${\mathrm{Zn}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Fe}}_{\mathit{x}}$Se, and very little change in ${\mathrm{Zn}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Co}}_{\mathit{x}}$Se. The exchange interaction has a much smaller effect on the band-gap energies.

The structure of the H-related complexes in p-type InP and in liquid encapsulated Czochralski semi-insulating InP:Fe has been studied from the vibrational absorption of their PH stretching modes. The acceptor complexes are produced by plasma hydrogenation so that PD modes have been investigated also. The study has first been performed at 6 K on the fundamentals and on the most intense of the first overtones. The trends in the frequencies and widths of the PH modes of the H-acceptor complexes for Be, Zn, and Cd acceptors are discussed and explained qualitatively. In InP:Fe, the PH intrinsic modes are sharper than those of the acceptor complexes indicating a weaker interaction with the environment. This study has been followed by the measurement of the temperature dependence of the frequencies and of the linewidths for increasing temperatures. The frequency shifts and the broadenings of the lines are interpreted by the temperature-dependent random dephasing of the vibration of the high-frequency oscillators in the excited state. The analysis shows that the PH mode in the acceptor complexes couples to TA phonons of the InP lattice while the one in the complexes involving a vacancy couples to a two TA phonon combination. The anharmonicity of the P-H bonds is comparable to the one in phosphine. A comparison of the anharmonicity parameters derived from the overtone measurements with those derived from the hydrogen isotope effects gives evidence of the interaction between the H atom and the lattice.

The amplitude of vibration of the D atom is smaller than that of the H atom and this explains why the interaction of the D atom with the lattice is smaller. This is the reason why the width of the PD modes is smaller than that of the corresponding PH modes. The splitting of some of the PH lines in samples subjected to a uniaxial stress has been studied. The splitting of the PH;Zn mode is in full agreement with a P-H bond along a 〈111〉 axis. The same 〈111〉 orientation of the P-H bond is also found from the splitting of a line attributed to an In vacancy ‘‘decorated’’ by a H atom (${\mathit{V}}_{\mathrm{In}}$(PH)). The splitting of the strongest line in InP:Fe leads to its attribution to a PH mode in a cubic center containing four H atoms (${\mathit{V}}_{\mathrm{In}}$(PH${)}_4$). The presence of this center seems to account for most of the hydrogen present in InP:Fe. Upon annealing of the InP:Fe samples, ${\mathit{V}}_{\mathrm{In}}$(PH${)}_4$ is a source of atomic hydrogen that can be trapped by other defects and it can leave partially hydrogenated In vacancies.

We report state-of-the-art first-principles calculations of the quasiparticle energies of prototype homopolar and heteropolar covalent semiconductors described in terms of the electron self-energy operator. The wave functions are calculated within density-functional theory using the local-density approximation and employing nonlocal, norm-conserving pseudopotentials. The self-energy operator is evaluated in the GW approximation. Employing the plasmon-pole approximation for the frequency dependence of the dielectric matrix ${\mathrm{\epsilon }}_{\mathbf{G},\mathbf{G}\prime }$(q,ω), its static part is fully calculated within the random-phase approximation (RPA) as well as by using a number of different models. All calculations are carried out employing localized Gaussian orbital basis sets. This will turn out to be very useful for detailed studies of the quasiparticle properties of more complex systems such as bulk defects including lattice relaxation and reconstructed surfaces with large unit cells or interfaces, which are otherwise computationally too demanding. Using an s,p,d,s* basis set of 40 Gaussian orbitals for Si, for example, yields already convergent results in excellent agreement with the results of a 350-plane-wave calculation in the corresponding plane-wave representation. Most of our results for Si, diamond, Ge, and GaAs are in very good agreement with experimental data and with available plane-wave GW calculations. To our knowledge, our results for SiC are the first quasiparticle energies reported so far for this important material of high current technological interest. Also in this case we find very good agreement with the available experimental data except for E(${\mathrm{L}}_{1\mathrm{c}}$). We believe that this deviation may be attributed to experimental uncertainties. In particular, we discuss and scrutinize the applicability of six different models for the static dielectric matrix ${\mathrm{\epsilon }}_{\mathbf{G},\mathbf{G}\prime }$(q,0) in the GW approximation ranging from the simple Hartree-Fock expression over diagonal models to nondiagonal models that take the local fields within the inhomogeneous electronic charge density into account. Some of the nondiagonal models are shown to yield results in very good agreement with the full RPA results.

Ab initio all-electron Hartree-Fock calculations within the molecular-cluster model are performed to analyze the role of N impurities, both isolated and complex, in type-IV semiconductors. The results are used to investigate the structural and electronic properties. For isolated impurities the N atom distorts in the 〈111〉 direction towards a vacancy leading to a final local ${\mathit{C}}_{3\mathit{v}}$ symmetry. The N atom forms ${\mathit{sp}}^2$ bonds with the host atoms and leaves an unoccupied N lone pair. For ${\mathrm{N}}_2$ both N atoms tend to form similar ${\mathit{sp}}^2$ bonds and move away from each other in the 〈111〉 direction without N-N bond formation. For the case of ${\mathrm{N}}_2^{+}$, localized N-N hole states are obtained. A detailed picture at the orbital level is given. These achievements should also be important for amorphous hydrogenated semiconductors.

The number of complete Rabi oscillations of the electron-hole density induced by square optical pulses with area 2π is studied using the coherent part of the semiconductor Bloch equations. It is shown that the frequency of the density oscillations depends strongly on the exciton binding frequency, detuning, and the Rabi frequency of the light pulse. The well known doubling of the density oscillation frequency is only observed if the Rabi frequency and exciton binding frequency are approximately of the same order of magnitude. However for strong and weak excitation the semiconductor exhibits properties of atomic systems.

We report on femtosecond time-resolved transmission and reflectivity measurements on bulk GaAs. Spectral-hole burning is observed, to our knowledge, for the first time in GaAs at room temperature. Carrier thermalization occurs within 200 fs and shows no significant dependence on excitation density or excess energy in the range from 2×${10}^{17}$ ${\mathrm{cm}}^{\mathrm{-}3}$ to 2×${10}^{18}$ ${\mathrm{cm}}^{\mathrm{-}3}$ and 35 meV to 90 meV, respectively. Calculations of the carrier dynamics are performed and include full dynamic screening of the carrier-carrier and the carrier LO-phonon interaction. The calculated thermalization times agree well with the experimental results. Negative transmission changes above the spectral hole are mainly caused by a complimentary increase of reflectivity, but not by an increase of absorption.

The structural and electronic properties of substitutional phosphorus and nitrogen impurities in ZnSe are studied using pseudopotential total-energy calculations. Substitutional phosphorus and nitrogen, in their neutral states, form shallow acceptors. In the case of phosphorus, the symmetry is lowered from ${\mathit{T}}_{\mathit{d}}$ to ${\mathit{C}}_{3\mathit{v}}$ by a Jahn-Teller distortion, and the hole is more localized on the phosphorus site. For nitrogen, however, the Jahn-Teller distortion is so small that the symmetry is approximately ${\mathit{T}}_{\mathit{d}}$, and the hole density is spread around nearby selenium-atom sites. For nitrogen in the positive charge state, two minimal-energy configurations are predicted: a stable state with approximately ${\mathit{T}}_{\mathit{d}}$ symmetry and a metastable state with ${\mathit{C}}_{3\mathit{v}}$ symmetry, which is 0.3 eV higher in energy than the stable one.

${\mathit{n}}_{\mathrm{H}}$ (Hall) and ${\mathit{n}}_{\mathit{C}\mathit{V}}$ (capacitance-voltage) electron-density data are compared in a Si-doped ${\mathrm{Al}}_{0.35}$${\mathrm{Ga}}_{0.65}$As sample having an ${\mathit{N}}_{\mathit{d}}$-${\mathit{N}}_{\mathit{a}}$=2.8×${10}^{18}$ ${\mathrm{cm}}^{\mathrm{-}3}$ net donor density. Data taken during low-temperature photoionization of DX centers and during temperature variations in the persistent photoconductivity regime (T<50 K) suggest that as the density of photoexcited electrons increases the Fermi energy rises until a Si-related ${\mathit{D}}^0$ donor bound state, resonant in energy with the Γ conduction-band valley, becomes populated. During the initial stage of isothermal capture transients, ${\mathit{n}}_{\mathit{H}}$ is found to decrease at a slower time rate than ${\mathit{n}}_{\mathit{C}\mathit{V}}$, since electrons are continuously supplied in the Γ valley by the depopulation of the ${\mathit{D}}^0$ level. During the final stage of the capture, when ${\mathit{D}}^0$ is empty, ${\mathit{n}}_{\mathit{C}\mathit{V}}$ and ${\mathit{n}}_{\mathit{H}}$ are found to coincide. The analysis based on an equilibrium distribution of the photoexcited electrons between the Γ valley and the ${\mathit{D}}^0$ level gives for this latter an energy of 40.6±0.5 meV above the Γ minimum (T=0). The possibility that the ${\mathit{D}}^0$ state is linked to the X secondary minima of the conduction band is discussed.

First-principles calculations by means of the linear-muffin-tin-orbital method were carried out of the electronic band structures, equilibrium lattice constants, energies of formation, and bulk moduli of ${\mathrm{Ga}}_{\mathit{x}}$${\mathrm{Al}}_{1\mathrm{-}\mathit{x}}$N alloys in zinc-blende-derived structures with a specific cation ordering. The miscibility and band-gap bowing of disordered alloys were studied using a cluster expansion method. Complete mutual solubility is predicted to occur at typical growth temperatures. The band gap is found to exhibit a weak bowing and to change from being indirect (Γ→X) for x=0 to direct at Γ at a concentration of x≊0.4.